Mr. Wheel of Fate: there are at least two versions of Sonic the Hedgehog 2 [U]. REV00 and REV01.
the codes that Tony Hedstrom has applies to rev. 01 of Sonic 2, which is the most common version. Rev. 00 was somewhat buggy and a Rev. 01 release was produced to fix bugs from Rev. 00.

all of the following game genie codes mentioned here will not work with Sonic 2 REV SC02.
yes, there was a third revision of Sonic 2 (aka. REV SC02), contained either the Sonic Classics or Sonic Compilation game.